A hologram is formed by recording interference fringes on a photosensitive material, which are produced by the interference of two types of lights (object light and reference light). When reference light is applied to this hologram, a diffraction phenomenon occurs, and the same wavefront as that of the original object light can be reproduced. Holograms can be divided into various types, depending on the recording forms of their interference fringes. Typically, they can be divided into surface relief holograms and volume holograms.
These holograms have such a characteristic that it is difficult to make duplicates thereof. Due to this characteristic, they are widely used in security applications. In security applications, surface relief holograms are generally used, which are formed by recording interference fringes by forming fine convexoconcaves on the surface of a hologram for layer. However, in recent years, due to the spread of hologram techniques and advances in counterfeiting techniques, counterfeit surface relief holograms have emerged and become a problem.
On the other hand, volume holograms are formed by recording interference fringes produced by the interference of lights, as fringes with different refractive indices and three-dimensionally in the thickness direction of a photosensitive material. Volume holograms are different from surface relief holograms in production method, and they are also absolutely different from surface relief holograms in visual effects, so that volume holograms make it easy to assay the authenticity. Also, they require high-level design techniques to reproduce the design, so that they make it quite difficult to duplicate holograms thereof for counterfeiting purposes. Therefore, volume holograms are used in applications such as design, security and optical element applications.
In the field of volume hologram production, dry-type photosensitive compositions for volume hologram recording, which can be produced on a large scale, have attracted attention. For example, there is a method that uses the refractive index difference between a photoradically polymerizable monomer and a binder resin which are contained in a photosensitive composition for volume hologram recording (for example, see Patent Literature 1, etc.) When interference exposure is carried out on a photosensitive composition for volume hologram recording formed into a film, radical polymerization is initiated in an area exposed to strong light, thus causing a concentration gradient in the photoradically polymerizable monomer and thereby a diffusive movement of the monomer from an area exposed to weak light to the area exposed to strong light. As a result, depending on the strength of the interfering light, the photopolymerizable monomer can be made dense or sparse, so that it appears as a difference in refractive index (hereinafter referred to as “refractive index modulation amount” or Δn); therefore, a hologram image is produced.
To achieve excellent recording performance during hologram recording, it is needed that a hologram recording layer obtained from a photosensitive composition for volume hologram recording has a high refractive index modulation amount (Δn) or excellent stability.
For example, a photosensitive composition for volume hologram recording is disclosed in Patent Literature 2, which uses a photoradically polymerizable monomer, a photocationically polymerizable monomer, a photopolymerization initiator, a sensitizing dye and a binder resin, which is a mixture a thermosetting resin and a thermoplastic resin. Patent Literature 2 tries to increase the heat resistance and mechanical strength of the hologram recording layer and to improve the refractive index modulation amount (Δn), by mixing the thermoplastic resin so as to account for 0% to 25% by mass of the total solid content of the photosensitive composition for volume hologram recording.
However, to produce a higher-luminance hologram image, a further increase in refractive index modulation amount (Δn) has been demanded. Also, to increase productivity, a photosensitive composition which provides high sensitivity during hologram recording, has been demanded.
In Patent Literature 3, to obtain an optical recording medium that enables highly-sensitive and highly-multiple recording, a photosensitive composition comprising a photoreactive material, an epoxy compound and a thiol group-containing compound is disclosed, as a photosensitive composition which is stable to humidity, which can be easily controlled over time in the form of liquid, which does not require high temperature curing, which is suitable for rapid production, and which is applicable to any types of forming methods. However, this photosensitive composition further contains a curing accelerator for reacting the thiol group-containing compound with the epoxy compound, and the thiol group-containing compound and the epoxy compound are thermally cured before irradiation with light. Therefore, in the photosensitive composition of Patent Literature 3, the thiol group-containing compound is not used as a chain transfer agent for the photoradically polymerizable monomer. Also, the photosensitive composition is used to form a recording layer that uses holography to record information. It is aimed at increasing the durability of recording media, and there is no description of increasing the sensitivity during hologram recording and increasing the refractive index modulation amount (Δn).
In Patent Literature 4, as a volume hologram recording material which is able to record/reproduce image information at an excellent signal-to-noise ratio and which is suitable for applications of holographic information recording, a volume hologram photosensitive composition comprising a polymer matrix, a polymerizable monomer, a photopolymerization initiator and a chain transfer agent is disclosed. In Patent Literature 4, the chain transfer agent is used to reduce noise caused by light scattering due to the polymer produced from the polymerizable monomer, and is also used to prevent a deterioration in records, by controlling the molecular weight of the polymer. In Patent Literature 4, there is no description of increasing the sensitivity during hologram recording and increasing the refractive index modulation amount (Δn). In Patent Literature 4, alkyl thiols are exemplified as the chain transfer agent. However, concrete examples disclosed therein are 1-butanethiol and n-dodecyl mercaptan only.